This invention relates generally to nuclear reactors and more particularly, to assemblies and methods for coupling piping within reactor pressure vessels of such reactors.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends by a bottom head and a removable top head. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
Boiling water reactors have numerous piping systems which are utilized, for example, to transport water throughout the RPV. Known boiling water reactors include a core spray system to deliver cooling water to the reactor core during a loss of coolant accident. The core spray system, including piping, downcomers, T-box assemblies, and core spray spargers, is used to deliver water from outside the RPV to inside the RPV. The core spray system provides water to the reactor core.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components exposed to high temperature water, such as structural members, piping, fasteners, and welds. The reactor components are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC.
Reactor internal piping, such as core spray spargers and T-box assemblies, occasionally requires replacement as a result of SCC or for other reasons. Replacing the spray spargers sometimes requires removing and replacing T-box assemblies, and core spray piping, including downcomers. Replacement of spray spargers, T-boxes, and downcomers is complicated by the limited working space, high radiation levels, and difficulties associated with welding to a highly radioactive shroud.
Internal reactor coolant piping must remain leak resistant when cool water is injected into the reactor vessel. The injection of cool water through the inside of the pipe results in thermal contraction of the pipe relative to bolted pipe couplings. The contraction of the pipe relative to the coupling bolts can result in loss of bolt tension. Loss of bolt tension reduces coupling seal seating force reducing the leak resistance of the seal.
To obtain elastic compliance to accommodate differential thermal expansion and still maintain seal seating forces, long slender Inconel® bolts of high strength are typically employed. The Inconel® bolting material is susceptible to intergranular stress corrosion cracking (IGSCC) at high stresses. Bolt preload must, therefore, be carefully controlled, limiting the seating forces that can be applied during coolant injection.